Electrosurgical generators are employed by surgeons to cut and coagulate the tissue of a patient. High frequency electrical power, which may be also referred to as radio frequency (RF) power or energy, is produced by the electrosurgical generator and applied to the tissue by an electrosurgical tool. Both monopolar and bipolar configurations are commonly used during electrosurgical procedures.
Electrosurgical techniques can be used to seal small diameter blood vessels and vascular bundles. Another application of electrosurgical techniques is in tissue welding, wherein two layers of tissue are grasped and clamped together by a suitable electrosurgical tool while the electrosurgical RF energy is applied. The two layers of tissue are then “welded” together.
At this point it is significant to note that the process of coagulating small vessels is fundamentally different than vessel sealing or tissue welding. For the purposes herein the term coagulation can be defined as a process of desiccating tissue wherein the tissue cells are ruptured and dried. Vessel sealing or tissue welding can both be defined as desiccating tissue by the process of liquefying the collagen in the tissue so that it crosslinks and reforms into a fused mass. Thus, the coagulation of small vessels if generally sufficient to close them, however, larger vessels normally need to be sealed to assure permanent closure.
However, and as employed herein, the term “electrosurgical desiccation” is intended to encompass any tissue desiccation procedure, including electrosurgical coagulation, desiccation, vessel sealing, and tissue welding.
One of the problems that can arise from electrosurgical desiccation is undesirable tissue damage due to thermal effects, wherein otherwise healthy tissue surrounding the tissue to which the electrosurgical energy is being applied is thermally damaged by an effect known in the art as “thermal spread”. During the occurrence of thermal spread excess heat from the operative site can be directly conducted to the adjacent tissue, and/or the release of steam from the tissue being treated at the operative site can result in damage to the surrounding tissue.
It can be appreciated that it would be desirable to provide an electrosurgical generator that limited the possibility of the occurrence of thermal spread.
Another problem that can arise with conventional electrosurgical techniques is a buildup of eschar on the electrosurgical tool or instrument. Eschar is a deposit that forms on working surface(s) of the tool, and results from tissue that is electrosurgically desiccated and then charred. One result of the buildup of eschar is a reduction in the effectiveness of the surgical tool. The buildup of eschar on the electrosurgical tool can be reduced if less heat is developed at the operative site.
It has been well established that a measurement of the electrical impedance of tissue provides an indication of the state of desiccation of the tissue, and this observation has been utilized in some electrosurgical generators to automatically terminate the generation of electrosurgical power based on a measurement of tissue impedance.
At least two techniques for determining an optimal amount of desiccation are known by those skilled in this art. One technique sets a threshold impedance, and terminates electrosurgical power when the measured tissue impedance crosses the threshold. A second technique terminates the generation of electrosurgical power based on dynamic variations in the tissue impedance.
A discussion of the dynamic variations of tissue impedance can be found in a publication entitled “Automatically Controlled Bipolar Electrocoagulation”, Neurosurgical Review, 7:2-3, pp. 187-190, 1984, by Vallfors and Bergdahl. FIG. 2 of this publication depicts the impedance as a function of time during the heating of a tissue, and the authors reported that the impedance value of tissue was observed to be near to a minimum value at the moment of coagulation. Based on this observation, the authors suggest a micro-computer technique for monitoring the minimum impedance and subsequently terminating the output power to avoid charring the tissue.
Another publication by the same authors, “Studies on Coagulation and the Development of an Automatic Computerized Bipolar Coagulator”, Journal of Neurosurgery, 75:1, pp. 148-151, July 1991, discusses the impedance behavior of tissue and its application to electrosurgical vessel sealing, and reports that the impedance has a minimum value at the moment of coagulation.
The following U.S. patents are also of interest in this area. U.S. Pat. No. 5,540,684, Hassler, Jr. addresses the problem associated with turning off the RF energy output automatically after the tissue impedance has fallen from a predetermined maximum, subsequently risen from a predetermined minimum and then reached a particular threshold. A storage device records maximum and minimum impedance values, and a circuit determines the threshold. U.S. Pat. No. 5,472,443, Cordis et al., discusses a variation of tissue impedance with temperature, wherein the impedance is shown to fall, and then to rise, as the temperature is increased. FIG. 2 of this patent shows a relatively lower temperature Region A where salts contained in body fluids are believed to dissociate, thereby decreasing the electrical impedance. A relatively next higher temperature Region B is where the water in the tissue boils away, causing the impedance to rise. The next relatively higher temperature Region C is where the tissue becomes charred, which results in a slight lowering of the electrical impedance. U.S. Pat. No. 4,191,188, Belt et al., discloses the use of two timers whose duty cycles are simultaneously and proportionately adjusted so that high frequency signal bursts are constantly centered about the peak power point, regardless of duty cycle variations.
Also of interest is U.S. Pat. No. 5,827,271, Buysse et al., “Energy Delivery System for Vessel Sealing”, which employs a surgical tool capable of grasping a tissue and applying an appropriate amount of closure force to the tissue, and for then conducting electrosurgical energy to the tissue concurrently with the application of the closure force. FIG. 2 of this patent, shown herein as FIG. 1 for depicting the prior art, illustrates a set of power curves which represent the electrosurgical power delivered to the tissue as a function of the tissue impedance. At low impedances, the electrosurgical power is increased by rapidly increasing the output current. The increase in electrosurgical power is terminated when a first impedance breakpoint, labeled as 1, is reached (e.g. <20 ohms). Next, the electrosurgical power is held approximately constant until proteins in the vessels and other tissues have melted. The impedance at which this segment ends varies in accordance with the magnitude of the RMS power. For example, where the maximum RMS power is approximately 125 Watts, the segment (B) ends at about 128 ohms. When a lower power is used (e.g., 75 Watts), the segment (C) may end at an impedance value of 256 ohms. Next, the output power is lowered to less than one half the maximum value, and the lower power delivery is terminated when a second impedance breakpoint is reached (2.048×103 ohms). Alternatives to using the impedance for determining the second breakpoint are the use of I-V phase angle, or the magnitude of the output current.
Based on the foregoing it should be evident that electrosurgery requires the controlled application of RF energy to an operative tissue site. To achieve successful clinical results during surgery, the electrosurgical generator should produce a controlled output RF signal having an amplitude and wave shape that is applied to the tissue within predetermined operating levels. However, problems can arise during electrosurgery when rapid desiccation of tissue occurs resulting in excess RF levels being applied to the tissue. These excess levels produce less than desirable tissue effects, which can increase thermal spread, or can cause tissue charring and may shred and disintegrate tissue. It would be desirable to provide a system with more controlled output to improve vessel sealing and reduce damage to surrounding tissue. The factors that affect vessel sealing include the surgical instrument utilized, as well as the generator for applying RF energy to the instrument jaws. It has been recognized that the gap between the instrument jaws and the pressure of the jaws against the tissue affect tissue sealing because of their impact on current flow. For example, insufficient pressure or an excessive gap will not supply sufficient energy to the tissue and could result in an inadequate seal.
However, it has also been recognized that the application of RF energy also affects the seal. For example, pulsing of RF energy will improve the seal. This is because the tissue loses moisture as it desiccates and by stopping or significantly lowering the output the generator between pulses, this allows some moisture to return to the tissue for the application of next RF pulse. It has also been recognized by the inventors that varying each pulse dependent on certain parameters is also advantageous in providing an improved seal. Thus, it would be advantageous to provide a vessel sealing system which better controls RF energy and which can be varied at the outset of the procedure to accommodate different tissue structures, and which can further be varied during the procedure itself to accommodate changes in the tissue as it desiccates.
An accommodation for overvoltage clamping is also desirable. In this regard, conventional overvoltage techniques use a means of clamping or clipping the excess overvoltage using avalanche devices such as diodes, zener diodes and transorbs so as to limit the operating levels. In these techniques the excess energy, as well as the forward conduction energy, is absorbed by the protection device and inefficiently dissipated in the form of heat. More advanced prior art techniques actively clamp only the excess energy using a predetermined comparator reference value, but still absorb and dissipate the excess energy in the form of heat.
U.S. Pat. No. 5,594,636 discloses a system for AC to AC power conversion using switched commutation. This system addresses overvoltage conditions which occur during switched commutation by incorporating an active output voltage sensing and clamping using an active clamp voltage regulator which energizes to limit the output. The active clamp switches in a resistive load to dissipate the excess energy caused by the overvoltage condition.
Other patents in this area include U.S. Pat. No. 5,500,616, which discloses an overvoltage clamp circuit, and U.S. Pat. No. 5,596,466, which discloses an isolated half-bridge power module. Both of these patents identify output overvoltage limiting for all power devices, and overvoltage limit protection is provided for power devices by using proportionately scaled zeners to monitor and track the output off voltage of each device to prevent power device failure. The zener device is circuit configured such that it provides feedback to the gate of the power device, When zener avalanche occurs the power device partially turns on, absorbing the excess overvoltage energy in conjunction with the connective load.
Reference can also be had to U.S. Pat. No. 4,646,222 for disclosing an inverter incorporating overvoltage clamping. Overvoltage clamping is provided by using diode clamping devices referenced to DC power sources. The DC power sources provide a predetermined reference voltage to clamp the overvoltage condition, absorbing the excess energy through clamp diodes which dissipate the excess voltage in the form of heat.
It would be advantageous as to provide an electrosurgical generator having improved overvoltage limit and transient energy suppression.